Revive your tired Dremel battery pack

It turns out there’s nothing more than six Nickel Cadmium AA rechargeable batteries inside of that cordless Dremel battery pack. Yep, standard rechargeable AA’s that you can buy most anywhere, and now you can revive that aging battery pack by following [Stuuf’s] guide. Since you’re already at it, a few more bucks will yield a real upgrade by using the superior Nickel Metal Hydride batteries which should yield around three times as much use between charging. We totally understand having a battery pack, since the shape of the case is part of the handheld tool, and it should be easy to interchange the battery as one unit. We just wish that the battery pack had been designed to have the AA cells swapped out by the user once they had reached the end of the line.

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21 thoughts on “Revive your tired Dremel battery pack”

When changing the type of batteries which are in the drill, you must take into consideration the characteristics of the charger.. It’s a better idea for most to use the exact type of battery which came with the battery pack initially. This is something which should not be taken lightly. It can lead to safety issues like burns or even a fire.

80 bucks at your local hobbie shop will buy you a good charger that can handle many types of batteries. If it were my batt-pack I would put LI-ion or li-po batts in the pack. RC cars have tought me lots about batteries and little of spelling

@paul,diad and adam… any charger for a battery pack this simple is little more than a transformer. we’re not talking electric cars here with $10,000+ battery packs that require extremely accurate charge curves. that said, paul wins; the rest of you are ninnies :P

Did this kind of hack with a cordless drill and spare laptop battery cells saved from a dumpster. (18650 4/3 A battery size) Works great as long as you have a way to charge them. Drill runs forever now and at a slightly higher voltage with just four half-dead lithiums.

Indeed, I agree with those who advised against changing batteries type: I gave a bunch of NiMH to the old man, he charged them with a NiCd charger, and it screwed them up within few charges (i.e. they would not hold a proper charge anymore, and they appeared permanently damaged). Meanwhile, I’ve been using the very same batteries in a proper slow NiMH charger, and they lasted much much longer.

Here’s a nasty little gotcha and how to avoid it: Sub-C NiCd cells used in drills often have the entire outer cylindrical shell attached to the negative terminal – including the outer rim at the edge of the +ve end. If the solder tab attached to + touches that outer rim, you get a short, lots of current, lots of heat and quite possibly a damaged cell, even if you correct the problem.

When I rebuilt my first drill battery and got three shorts, I corrected the problem with 3 layers of electrical tape under each of the solder tabs. Only later did I find out that the battery store can give you cardboard washers made to go around the +ve terminal and under the solder tabs, thus preventing the problem.

If you get Battery Boys to rebuild your battery pack for you (which is costly), some of the kids that work there don’t even know this. My friend got two DeWalt packs ‘professionally’ rebuilt that way but they never worked so he threw them in a box. A year later, armed with my new knowledge, I opened them up and found them a mass of repairman-induced shorts and failed cells. $150 wasted.

What I’m planning to build:
– modify each of my battery packs so that each end of each cell comes out to a DB25 connector mounted on the frame of the pack. For an 18V pack, that’s 12 cells and thus 13 nodes.
– build an Arduino-based pack analyzer that I can, at will, plug into that connector and analyze the state of charge of each cell in real time. While using the drill if I want. That will keep me from discharging a battery pack past the point where the weakest cell goes into reversal. If I stop when the lowest-capacity cell drops to 9V, none of the cells will be damaged and I can replace that lowest-capacity cell if I want.
– The electronics will include some analog switches so it can connect various cells to the six analog inputs of the Arduino, and also include an LCD display (the 10-bit ADC resolution is sufficient!). My intention is to display the state of each cell in a string of 12 hex digits with 0 being 0V and F being a fully-charged cell. If a cell is ever driven into reversal, its character would be highlighted. Thus a string of fully charged cells would display a string of 12 F characters. With software calibration I can pick a digit which corresponds to 0.9V optimum fully-discharged state and have it beep or flash or something to alert me that I should stop there and switch packs.
– Ordinary fast charging can be done with the regular drill charger, but if I find that it is over-charging some cells I could pretty easily have my accessory stop the charger.
– For cell testing and maintenance I can use my Lacrosse Technologies BC-900 battery charger / analyzer / restorer (originally recommended by HaD a few years ago) to rate each cell, thus knowing which ones need replacing, and how bad off they are.
– Since all four of the charging positions in the Lacrosse charger share a common ground, I add a second connector to the side of the battery pack, this one a Molex six-pin wired to conduct operating current (probably 12AWG). The point is to divide the series string of 12 cells into four strings of 3 cells each. Thus I can analyze, test or charge four cells at once instead of just one. If the nodes of the battery pack can be labelled 0 (battery pack negative) through 12 (battery pack positive), then the Molex connector breaks nodes 3, 6 and 9, bringing both ends out to where, most of the time, the breaks are bridged by a shorting link for normal operation. However, if I want to isolate part of the battery pack for analysis or maintenance, then I can.

I hope my attempt to describe this without sketches is somewhat comprehensible? And if you’re wondering why I would go to all that effort, two reasons:
– I have awful luck with my battery packs.
– … I *can*!

Here’s a question about my LaCrosse BC-900: I killed one of the four channels, but only the DISCHARGE function, which dumps through a string of chip resistors. All four channels charge, no problem. Has anyone traced the circuit yet and knows the transistor or FET that controls the resistive load? I haven’t gotten a round tuit yet.

yeah, people don’t realize this very often. NiMHs are not superior in every respect. For most consumer gear, NiMH is strictly better–at normal temperatures and moderate discharge rates, they get longer life and have higher energy density. Unfortunately, it turns out, at high power drain and for devices that use large spikes of power, NiMH actually cannot deliver as much power and will discharge faster than NiCds. In addition, NiCds (some grades, at least) can have wider temperature ranges. There’s a reason they don’t let you swap out the batteries–unless you special order the correct type of NiCds (just because it’s the same shape does /not/ mean they’ll work. . .really, guys) it won’t perform to original spec. And NiCd cells aren’t easy to find these days, given that most applications use NiMH cells. . .

Did the exact same thing with my dad’s DeWalt 18V cordless drill. The cells inside the battery pack are 4/5 sub C size, originally NiCd with 1200 mAh rating. Replaced with NiMH at 3300 mAh rating (there were much higher rated batteries, but the price got steep kinda fast).

I attempted to connect the tabs using “conductive glue” which proved to be too brittle for my inexperienced hands, so I found a place with a pulse welder that did the connecting for me for $30. So for about $80 I got a battery pack with three times the capacity of the original.

To inform you: NiMH *should* be charged on a different apparatus from NiCd, but if you withdraw a NiMH battery as soon as it’s full your NiCd charger won’t mess it up too quickly — it’s the trickle that’s different between the two.

Not all NiCads are created equal, and not all NiCad chargers are either. If you look at NiCad battery types you will find (among others) types R – rapid charge, H – high temperature, S – super high capacity/rapid charge, E – high capacity. Charging rates and internal resistance are only a couple of the specs used to spec the charging rate(s) of the chargers. Battery packs also may have an internal thermostat or other sensor to signal charging condition to the charger, and changing the battery type in the pack will affect the temperature profile in the pack. Anything more than a trickle charge wall-wart will have some electronics in the charger matched to the specific battery type(s) that the charger is designed to be used with.

Attempting to charge the wrong type battery with a fast charger WILL create a fire or explosion hazard. It may not happen right away, but it will happen. Leaving a mismatched pack in a fast charger for an extended time is a fire waiting to happen.

An over charged pack (because of a mismatched charger) may also catch fire or explode under use, or cause a switch failure. In a variable speed power tool, this failure can either be an open or a short. A dead short across a fully charged battery will make bad things happen. Quickly! Look inside a power tool battery, and you also see that one of the straps welded between the batteries will be either a fusable link, or other device to protect the battery against a dead short.

If you’ve ever seen a high capacity battery self destruct, you would really try to avoid that happening while holding one.

What’s hard to find is an AA cell charger that charges each one independently. I have two. One is a second revision Rayovac Renewal charger that does Renewal alkaline, NiMh and NiCd. The original Renewal charger only works with Renewal cells. The other one is a Kodak K605.

I have an Energizer charger that’s faked up with four LEDs but only charges pairs of cells. Kinda useless to charge three AAA cells for LED flashlights.